The basic goal of any hearing aid (HA) system is to improve speech intelligibility (SI) in conversational situations. Current HAs succeed, to a large extent, in achieving this goal, when conversation takes place in acoustically quiet surroundings. However, in complex acoustic situations, e.g. with disturbing noise sources and/or reverberation, existing HAs are still unable to improve SI sufficiently.
Rather than trying to optimize SI directly, existing HAs tend to process the microphone signals to maximize other quantities which are assumed or known to correlate with intelligibility. For example, HA noise reduction systems tend to maximize a signal-to-noise-ratio (SNR) because a) this is practically possible, and b) it is known that increasing SNR tends to increase SI. The drawback of this approach is that it is indirect/implicit: increasing SNR tends to increase SI, but there is not always a clear one-to-one map.